1. Field of the Invention
This invention relates generally to mooring systems for offshore vessels and Floating Production Units (“FPUs”) such as Floating Storage and Offloading vessels (“FSOs”), Floating Production Storage and Offloading vessels (“FPSOs”), Floating Storage Drilling Production and Drilling Units (“FPDSOs”) and in particular to turret mooring arrangements, or systems, where a turret is rotatably supported on the vessel and where the turret is fixed to the sea bed by anchor legs so that the vessel can weathervane about the turret.
2. Description of the Prior Art
Turret mooring systems have been used for some time for FPUs and especially with FPSOs. FPSOs are production platforms typically constructed by reconfiguring existing tanker hulls. FPSOs are the most useful of FPUs in terms of water depth and sea conditions due to their variation in moorings and ship shape configurations. FPSOs are either spread moored (anchored directly to the seafloor and unable to completely weathervane and rotate around a center point of mooring), or they are attached to the seafloor via an internal or external rotatable turret that is moored to the seafloor for 360° weathervaning capability of the vessel. FIG. 1 is an illustration of a prior art turret moored FPSOV with the turret connected to the sea floor by groups of anchor legs L and risers R running from the sea floor to the turret for rotatable coupling to vessel pipes which run to storage holds.
FPSOs compete with other kinds of floating production units such as semi-submersibles, spars, and tension leg platforms. These other systems generally do not have large product storage capacity like FPSOs, but they do have the advantage of easily handling a large number of risers (the flexible pipes and control umbilicals connected between the production unit and subsea wellheads). Large numbers of risers are required for subsea oil fields when it is not desirable to use subsea manifolding connecting several wells together. The number of risers can be from the twenties to ninety or even more. The spread moored FPSO has the advantage of large product storage capacity and also has the space capability for large numbers of risers. One main disadvantage of the spread moored FPSO is the reduced availability for tandem offloading due to occasional bad weather conditions preventing a safe approach of the shuttle tanker to connect to the FPSO. In many locations the rough weather direction changes and can also cause undesirable rolling motions of the vessel that are problematic to the process equipment and to the crew. The competitiveness of all of the above floating production units depends on their advantages and disadvantages.
As mentioned above, the present invention is directed to a turret mooring arrangement, and in particular to a rotatably mounted turret of large diameter for the purpose of accommodating a large number of risers and for providing other advantages resulting from a large diameter geostationary turret. Such advantages are summarized below.
Prior turret mooring arrangements are known in the art that include turrets of small to moderate diameter where the problems associated with vessel hull deflections are considered. A moonpool (a cylindrical tube extending from top to bottom through a vessel hull) is required to contain and usually support the turret bearing and turret shaft. Flexure of the vessel hull due to sea conditions can cause undesirable structural deflections in the moonpool at the foundations for the turret bearings. This effect can be substantial and detrimental for large moonpool diameters, and unless steps are taken to mitigate such effects, the turret bearings will suffer from high concentrated loads.
Prior turret designers have sought to minimize turret diameters due to requirements of roller bearing assemblies requiring flat machined surfaces not exceeding a predetermined diameter. In such arrangement, designers have sought to isolate the flat bearing races with various elastic elements and apparatus in an effort to accommodate hull deflections. Other designers have attempted to provide bearing wheel and rail arrangements for vessel-turret designs. A few of the prior art attempts to solve the problem of vessel hull deflection as it affects bearing operation is presented below.
Norwegian Patent No. 165,285 shows a structural suspension that attempts to provide a satisfactory load distribution around a bearing wheel track that may not be flat. Independent radial arms are disclosed to which vertical and radial load rollers are attached. The radial arms attach to a circular ring that twists to add to the flexibility of the bending beam deflection of the arms. This concept is limited in load carrying capacity and limited to relatively small turret diameters.
U.S. Pat. No. 5,052,322 to Poldervaart illustrates a bearing fixed to a rigid ring that does not follow deformations of the hull of the ship. A cylindrical tube supporting the rigid ring tends to flex with the vessel hull while the bearing and turret remain relatively isolated from hull deflection. The benefits of this design diminish as the moonpool (or turret insert tube) diameter and hull deflections increase.
U.S. Pat. No. 5,515,804 to Pollack shows internal and external turret bearing arrangements with a generally rigid upper mount including a resiliently deflectable support structure that includes a plurality of elastomeric shear pads. These arrangements are also difficult and expensive to scale up to large diameters due to the proportionally increasing size and shear motion capacity of the shear pads.
U.S. Pat. No. 5,359,957 to Askestad illustrates radial bearing arms connected to a substructure in the turret which provide individual suspension and can absorb unevenness and deformations in the bearing. Rollers attached to the ends of the radial arms support the turret load. This design is also limited in load carrying capacity by the difficulty of attaching large numbers of rollers for high load capacity.
U.S. Pat. No. 5,517,937 to Lunde shows a turret arrangement for accommodating many risers in which the riser tubes are arranged at an angle to minimize the bearing diameter to about eight meters or less while the bottom diameter of the turret is made large in diameter to accommodate the necessary spacing of the risers below the turret. Minimizing the bearing diameter is one way of mitigating the effects of the previously mentioned deflections, but construction complexity and other disadvantages such as limited equipment space inside the turret result from this arrangement. As the numbers of risers increase, their weight eventually overcomes the available capacity of the smaller bearing diameters.
U.S. Pat. No. 5,860,382 to Hobdy illustrates a turret with radial bearing rollers arranged with spring assemblies that allow for unevenness of the radial wheel rail and maintain roller contact with their rail. This arrangement of turret and bearing is suitable for risers numbering thirty to forty, but may not be practical for a much larger quantity of risers. The limitation of larger turrets of this design is the low flexibility of the tube-shaped turret structure. The turret is vertically shear-stiff, and the wheel and rail system must therefore be designed for significantly increased loading per wheel to accommodate the out-of-flat condition of the vertically loaded wheel rails.
U.S. Pat. No. 6,164,233 to Pollack describes bearing devices that include hydraulic cylinders and pistons to support vertical loads that are arranged to accommodate vessel hull deformations.
U.S. Pat. No. 6,263,822 to Fontenot shows elastomeric pads arranged radially and vertically around the main bearing which rotatably supports a mooring turret. This arrangement for shear and compression of elastomeric pads serves to compensate for hull deflection at the main bearing. The elastomeric pads accommodate vertical and radial deflections of the hull. This design is also expensive and may be difficult to scale upward to a large size.
U.S. Pat. No. 6,269,762 to Commandeur illustrates a bogie wheel bearing support structure mounted on top of a moonpool tube that extends above the connection to the vessel hull to isolate the bearing structure from the hull deflections. Commandeur also shows elastically deformable elements (rubber filler) beneath the bogie wheels to help even out the load on the wheels. The very tall moonpool tube also serves to isolate radial hull deflections from the bearing assemblies.
The advantages of this invention will be more apparent by comparison to prior art turrets.
FIG. 2 shows a prior art large turret capable of supporting 43 risers that was supplied for a Petrobras Field Development offshore of Brazil. The illustration is of the turret parts loaded on a barge B for transport from the fabrication yard. The complex arrangement of the lower turret T can be seen in which the turret structure and all of the riser guide tubes are tapered toward the top end in an effort to reduce the upper bearing diameter. The turret structure is of rigid construction.
FIG. 3 is a drawing of a prior art turret supplied by SOFEC, Inc. for an offshore oil field in the South China Sea. The turret 200 has a cylindrical tube structure that is relatively rigid in bending and shear. The upper bearing structure and the turret are rigid in the radial and vertical directions. A spring suspension system supporting the upper bearing 202 in combination with a heavily reinforced bearing support 204 structure allows structural deflections of the vessel at the turret insert tube (moonpool) without overloading the bearing. The bearing is a three-row roller bearing mounted in a manner similar to the apparatus of U.S. Pat. No. 5,356,321 to Boatman. This turret arrangement is typical of many in the single point mooring industry utilizing a combination of a lower bearing 208 near the vessel keel with an upper bearing 202 located near the main deck of the vessel.
FIG. 4 is a drawing of a prior art turret designed and supplied by SOFEC, Inc. for an oil field offshore of Brazil. An upper bearing system, located near the main deck of the vessel, includes a radial wheel/rail bearing 210 and an axial wheel/rail bearing 212 to provide rotational support between the turret and the vessel. No lower radial bearing was provided. A wheel and rail bearing system was provided for the vertical load to withstand large loads, because hull deflections concentrate the load onto only a fraction of the wheels. The vertical load rollers were designed with sufficient excess capacity per roller to carry the total load on only a portion of the total number of rollers. Radial wheels mounted on springs that spread the load over many radial wheels accommodates the radial deflections. The turret is stiff in both the radial and vertical directions.
For small diameter turrets, an axial roller bearing assembly can be provided between the turret and the vessel. Such roller bearing assemblies require that the bearing races be flat, machined surfaces. Such races have in the prior art often been isolated from ovaling due to vessel sagging and hogging by various elastic arrangements between a lower bearing race and the vessel. As the diameter of the turret becomes very large, roller bearing assemblies are not feasible due to the inability to machine flat surfaces for the very large diameter. Wheel-rail assemblies can be installed between the turret and the vessel, as described above, but for very large turrets carrying a very large number of risers, the forces on certain wheels due to the sagging or hogging of the vessel can become so large as to make it impractical to provide a very large turret for accepting a very large number of risers. The above very large number of risers connotes a number of from 40 to 120 risers.
Summing up, the problems for designing a very large turret (VLT) in the past have been either of vertically and radially stiff construction combined with various expensive devices to isolate the bearing, or they are limited in their range of diameter and load carrying capacity. The problems associated with a relatively inflexible structure limits the economic benefits of a large diameter turret, requires larger bearing capacities, and tends to reduce the wear life of the bearings.
3. Identification of Objects of the Invention
A primary object of the present invention is to provide an economical turret arrangement that has inherent structural flexibility, thereby making practical a large diameter main bearing that supports a very large turret.
Another object of the present invention is to provide an economical large diameter turret mooring arrangement for an FPSO that will accommodate a very large number of risers (either flexible non-metallic pipe or rigid steel pipe flow lines) where the large number of risers greatly exceeds those presently known in the art.
Another object of the present invention is to provide a practical turret configuration of sufficient size that allows a weathervaning vessel to be used as a floating production unit (FPU) with at least as many risers as can be connected to a non-weathervaning FPU such as a spread moored ship-shaped vessel or a semi-submersible vessel.
Another object of the present invention is to provide a wheel and rail bearing arrangement for a very large turret (VLT) frame configuration that has sufficient flexibility so that vessel hogging and sagging deflection causes a maximum load per wheel to increase not more than preferably about 50 percent greater than would occur with the rails in a perfectly flat plane, and not exceeding 150 percent greater than would occur with the rails in a flat plane.
Another object of the present invention is to provide a turret with a flexible structural frame configuration that allows a sliding-type lower bearing of a diameter greater than 12 meters diameter to be used near the vessel keel elevation in combination with an upper bearing greater than about 14 meters diameter located near the vessel main deck.
Another object of the present invention is to provide a turret with a flexible structural frame configuration with elastomeric bumper pads attached to the lower turret near the vessel keel elevation in combination with an upper bearing greater than about 14 meters diameter located near the vessel main deck.
Another object of the present invention is to provide a turret with a flexible structural frame configuration that allows the optional installation of protective riser tubes between the chain table and the main deck without appreciably increasing the stiffness of the turret frame.